Long-haul optical communication systems, e.g., optical communication systems spanning a distance of greater than about 600 kilometers, suffer from signal attenuation resulting from a variety of factors, including scattering, absorption, and bending. To compensate for attenuation, long-haul systems may include a series of optical amplifiers or “repeaters” spaced along the transmission path between a transmitter and a receiver. The amplifiers amplify the optical signal in a manner allowing reliable detection at the receiver. Usually, multiple repeaters are positioned along a single fiber optic transmission link, with numbers reaching more than a hundred per link in submarine systems. Power efficiency of repeaters, particularly those used in submarine applications, is quite important. For terrestrial systems, increasing efficiency is crucial for reducing amplifier size and cost, including material and operating costs. For submarine systems increasing efficiency is important to minimize the cost of labor of installing multiple repeaters in remote, difficult to reach, locations and in supplying energy to the repeaters in such locations.
Erbium doped fiber amplifiers (EDFAs) have proven particularly useful in long-haul systems. EDFAs include C-band EDFAs and L-band EDFAs which are used to amplify different optical bands, denoted as C-band and L-band. C-band usually includes wavelengths from 1530 nanometers (nm) to 1565 nm and L-band usually includes wavelengths from 1565 nm to 1625 nm. Both C-band and L-band features the lowest attenuation of commonly used optical transmission bands, the exact wavelength of the lowest attenuation depends on fiber design and can be in either C or L band. EDFAs may amplify only C-band signals (referred to as a “C-band EDFA”), only L-band signals (referred to as an “L-band EDFA”) or both C-band and L-band signals (referred to as a “C+L EDFA”). Generally, each EDFA includes nearly independent C-band and L-band amplification portions—i.e., the amplifier is a combination of two EDFAs: one C-band EDFA and one L-band EDFA, with economies taken in the form of shared components within the C+L EDFA. In a C+L EDFA, the input optical signal is usually split between C-band and L-band using a device such as a C+L demultiplexer or splitter. The C-band and L-band signals are independently amplified and recombined using a C+L multiplexer or combiner. Physically, the splitter and the combiner may be similar devices and the name simply denotes the functionality assigned to the device.
Usually, an EDFA is used to produce gain having a particular spectral shape over the signal wavelength band—i.e., over the amplification band or range of the device. The spectral shape is usually “flat” inasmuch as the amplification across the wavelength band of the device is either similar or varies linearly with the signal wavelength. The exact amplification shape may be achieved through the use of a Gain Flattening Filter (GFF).
Several types of GFF exist. One type of GFF uses Short Period Fiber Bragg Grating (SP-FBG) that is able to provide very accurate shaping of the optical signal over the amplification band. Such accuracy is advantageous in long links that characterize submarine communication systems where the number of repeaters is large and errors in the EDFA gain shapes are undesirable. One feature of SP-FBG filters is that the filtering function is performed by redirecting unneeded light, including both signal and amplified spontaneous emission (ASE) noise in the backward direction—i.e., in a direction opposite the direction of propagation of the optical signal. Usually, the back propagated light is undesirable for the upstream EDFAs and is blocked using an isolator positioned before the SP-FBG filter.
There is therefore a need for systems and methods of reducing the power demand presented by amplifiers along difficult to access optical transmission lines such as submarine transmission lines. There is also a need for systems and methods of beneficially recovering the energy present in the optical signals reflected by filters such as SP-FBG filters, particularly along difficult to access optical transmission lines such as submarine transmission lines.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications and variations thereof will be apparent to those skilled in the art.